1. Field of the Invention
The present invention relates to optical media for recording information, and more specifically to an optical information recording disk having a pair of plates which form a space therebetween in which at least one of the plates carries an information bearing layer on the inner surfaces thereof.
2. Description of Background Information
Optical disks are popularly used for recording various information in which the information is recorded in the form of a large number of minute pits on the information bearing layer. The DRAW (Direct Read After Write) disk is an example of optical information recording disk in which the reading of the recorded information directly after writing is enabled.
Since the information is carried by the DRAW disk in the form of sequential arrangement of the pits, it is quite important to protect the information bearing layer in which the information is held as the series of pits.
To meet this condition, a conventional type of optical disk has been developed which used a pair of disk-shaped substrates at least one of which is transparent and carries the information bearing layer. Both of the substrates are placed to face each other and combined together by sandwiching an adhesive agent therebetween. Thus, the information bearing layer carried by the substrate is formed between the inward surfaces of the substrates.
On the other hand, for improving the recording characteristics of the DRAW disk, it has been required to raise the sensitivity of the information bearing layer so that the error ratio of the DRAW disk is reduced.
In the case of the above described dual substrate optical disks of conventional design, a drawback is that the information bearing layer is likely to become corroded by the adhesive agent used to combine two substrates when the information bearing layer is made extremely sensitive.
Furthermore, another conventional type of optical disk is also been developed in recent years. This type of DRAW disk comprises a pair of disk-shaped substrates each of which has a center bore and at least one of which substrate is transparent and carries the information bearing layer on the main surface thereof; an inner annular spacer aligned with the center bore and placed between the inner circumferential portion of the main surface of the substrates; an outer annular spacer positioned coaxially to the inner annular spacer and placed between the outer circumferential portion of the main surface of the substrates, these inner and outer annular spacers being respectively provided with circular grooves on cylindrical surfaces thereof so that the circular grooves face each other and having U-shaped cross-sections in the radial direction thereof; and a circular partition which is supported by means of the outer and inner annular spacers each having the circular groove so as to insert the outer and inner circumferential portions of the partition into the oppsite grooves of the spacers respectively. In this case, both of the substrates are placed to face each other and combined together through the inner and outer annular spacers so as to make a space therebetween. Thus, the outer circumferential portion of the circular partition is slidably fitted in the inward circular groove of the outer annular spacer, and the inner circumferential portion of the circular partition is also slidably fitted in the outward circular groove of the inner annular spacer, so that the circular partition separates the space existing between the substrates into two spaces. The information bearing layer formed on one of the substrate is opposite to an inward surface of the other substrate through the circular partition and the spaces.
In the case of this optical DRAW disk having gaps or spaces between the substrates, the corrosion of the information bearing layer caused by the adhesive agent, as the above described problem in the prior art, is almost avoided since the adhesive agent hardly touchs the information bearing layer except for the circumferential portions of the inward surfaces of the substrates.
However, this optical DRAW disk having such annular spacers is weak, because the substrates are slidably supported by only means of the inner and outer annular spacers having specific forms at the inner and outer circumferential portions thereof. Particularly, the outer annular spacer sandwiched between the substrates is easily fragile, because a shearing stress caused by the modification of the entire DRAW disk is concentrated on the bottom of the inward circular groove of the outer spacer having the U-shaped cross-section when such DRAW disk is loaded for playing in the player.
Thus, it is a problem concerning the strength of the DRAW disk in this conventional technique that the substrate must be manufactured with thickness enough to maintain the stiffness of the entire DRAW disk including its annular spacers, so that the apparatus for playing such DRAW disk should be made relatively large in size. For example, there appears a DRAW disk having a thickness of 8 mm in the market.